BANGKOK -- Cemex S.A. is poised to inject about $300 million in TPI Polene PCL, a deal that will give the Mexican giant a controlling stake in the Thai cement company.

Under an agreement expected to be signed Sunday, the world's third-largest cement company will acquire at least 50% of TPI Polene's equity. Cemex will also assume a management role at the Thai cement company together with the Leophairatana family, which founded TPI Polene, according to people familiar with the negotiations.